1. Field of the Invention
The present invention relates to face identification devices and to technology for registering a face in a face identification device.
2. Description of the Related Art
In a face identification device based on face recognition technology, an image is used for face registration (teaching) and identification, and therefore differences in imaging conditions of the image used for registration and identification (illumination, face direction, expression, age, etc.) largely affects identification accuracy. In order to secure robustness with respect to differences in such imaging conditions, it is generally preferable that image registration is performed using a large number of images imaged under various conditions.
However, imaging under various conditions imposes a heavy burden on users who perform face registration. Further, it is difficult to achieve desirable imaging conditions. Furthermore, there is a need to readily perform face registration using a small number of images (for example, ID photos adhered on licenses, passports, etc.).
Conventional methods and devices have attempted to address these problems in various ways. First, there is proposed a method in which surface reflectivity is used for face identification by estimating the surface reflectivity of a face from an input face image and an average face three-dimensional shape model, or an image under arbitrary illumination conditions from which the surface reflectivity is generated (see, Japanese Patent Application Laid-Open No. 2002-24830). This method corresponds to illumination change by using surface reflectivity, which is a parameter less susceptible to illumination conditions.
Although JP 2002-24830 considers surface reflectivity (under various illumination conditions) when performing registration, the method lacks versatility because it cannot correspond to other than fluctuation of the illumination conditions and the feature value used for face identification is limited to the surface reflectivity, etc. Furthermore, the face three-dimensional shape model is used and a conversion process based on the image is performed; therefore, there is a disadvantage that an amount of memory capacity and calculation cost become enormous.
Next, in face image recognition according to a subspace method, there is proposed a method in which an autocorrelation matrix R′ of an image obtained under other illumination conditions is virtually made by adding a covariance matrix C in an illumination standard space which is a fluctuation element model the of illumination conditions with respect to an autocorrelation matrix R of a registered image obtained under certain illumination conditions (refer to Japanese Patent Application Laid-Open No. 2005-202673, and Kenji Matsuo and two others, “Face Image Recognition Suppressing Illumination Fluctuation Due to Virtual Part Space,” TECHNICAL REPORT OF IEICE, THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, Jun. 18, 2004, Vol. 104, No. 125, pp. 25-30).
JP 2005-202673 is specialized to image recognition technology using the subspace method and therefore lacks versatility. Furthermore, since an eigenvalue problem cannot be solved in one dimension (in other words one sheet of registered image), a plurality of sheets of the registered images is needed (for example, 50 sheets of images are used in the above-mentioned document written by Matsuo, et al.). Even when the illumination condition is limited to one kind, if several tens of sheets of images need to be imaged, the amount of processing required remains relatively the same and the amount of memory and calculation costs increase.